Recently, chemical amplification type DUV (deep ultra violet) photoresists have been investigated as a means for achieving high sensitivity in micro processes for preparing a semiconductor device. These photoresists are generally prepared by blending a photoacid generator with photoresist polymer matrix macromolecules having acid labile substituents in an organic solvent to form a photoresist composition.
According to the reaction mechanism of such photoresists, the photoacid generator generates acid when it is irradiated by the light source, and substituents on the main chain or branched chain of the matrix macromolecule in the exposed portion react with the generated acid to be decomposed or cross-linked, thereby considerably altering the polarity of the macromolecule. As a result, there is a solubility difference in the developing solution between the exposed area and the unexposed area. For example, in case of a positive photoresist, the main or branched chain of the matrix macromolecule is decomposed by acid in the exposed area and is removed by being dissolved in the developing solution. On the other hand, in the case of the unexposed area, the original structure of the macromolecular compound is maintained without being dissolved in the developing solution. As a result, the image of a mask is formed on the substrate as a positive image. In the lithography process, resolution depends upon the wavelength of the light source--the shorter the wavelength, the more minute the pattern that can be formed.
A suitable photoresist polymer generally requires excellent etching resistance, heat resistance and adhesiveness, and, in particular, when a photoresist is used for lithography processes employing ArF light source, it should be developable in 2.38% aqueous tetramethylammonium hydroxide (TMAH) solution. However, it is very difficult to synthesize a polymer that satisfies all these requisites. For example, a polymer having a polyacrylate main chain can be easily synthesized, but it has poor etching resistance and difficulties in the developing process. Etching resistance can be enhanced if an alicyclic monomer is introduced into the main chain. However, it is very difficult to synthesize a polymer having a main chain comprised of all alicyclic monomers.
Research to solve the problems described above has been widely performed in the past. In the case of forming an ultramicro pattern of 0.10 micron or less, the thickness of the photoresist layer must be 0.3 micron or less, but no photoresist has been developed to date which is resistant to etching gas in such a thin layer. In other words, any photoresist composition developed up to the present shows insufficient adhesiveness to the substrate and etching resistance when it is applied as a photoresist layer for a semiconductor element requiring resolution of 0.1 .mu.m or less.